The evidence comes from electrons and their more massive cousins, muons and tau leptons. According to the Standard Model, these three particles should behave like differently sized but otherwise identical triplets. But three experiments have produced growing evidence—including results announced in just the last few months—that the particles react differently to some as-yet mysterious influence. The findings are not yet conclusive, but if they hold up, “it would be a complete revolution,” says California Institute of Technology theorist Mark Wise.